Adhesive Research (Types of Adhesives, Adhesive Behaviour, Adhesive Design Tips)
- Tommy Tran
What is a structural adhesive? https://www.intertronics.co.uk/2018/04/what-is-a-structural-adhesive/
A structural adhesive is an adhesive which “hardens” or cures into a material capable of holding two or more substrates together, bearing the forces involved for the lifetime of the product. It is often termed a “load-bearing” adhesive. The product may undergo shock, vibration, chemical exposure, temperature excursions or many other types of potentially weakening or destructive agents, and still be bonded. The substrates may be the same, or quite different, ranging from metals, plastics, glass, rubbers, ceramics or composites. The parts may be under constant load, or intermittent loads and shocks.
What are epoxy adhesives? https://www.hotmelt.com/blogs/blog/adhesive-academy-epoxy-explained
Withstands heat, water and harsh chemicals.
Has highest adhesion and flexible enough to be molded into different shapes
Durable; withstand heavy loads – excellent structural adhesives
Comes in one-component or two-component systems (difference is in curing temperatures)
One-Component Epoxy Adhesives are cured between 250 – 300 degrees Fahrenheit
Formulated without a catalyst and cures faster than two-component epoxy systems
Excellent adhesive properties
Resistant to harsh external environments
Excellent alternative to welding and rivets
Two-Component Epoxy Adhesives
Cured at lower temperatures compared to one-component systems
Curing done with catalyst and process can be accelerated by heat
Cross-linking in the polymers increases during this heat acceleration process to give the epoxy superior properties
Choosing the Right Epoxy Adhesive
Work life – how long it takes for the epoxy adhesive to harden and dry
How long you have to manipulate the adhesive; you don’t want the epoxy adhesive to harden and dry before your job is done. You also don’t want your materials to shift and slide after your job is complete because your epoxy adhesive has not dried and hardened in time.
Cure time – length of time it takes for internal chemical reactions to complete and the epoxy to reach its full strength
Type of substrates
Some special epoxy adhesives have been developed
Colour
Highly visible jobs should have epoxy adhesives blend in with the substrate
Advantages of Adhesives Over Mechanical Assembly https://www.masterbond.com/techtips/advantages-adhesives-over-mechanical-assembly
Structural adhesives now offer a new solution to replace traditional mechanical assembly. These kinds of adhesives are user-friendly and allow for the permanent assembly of similar and dissimilar substrates. Some of the benefits of using adhesives in your assembly application include:
Bonding a wide variety of substrates
Even distribution of stress on the load
Protection from corrosion
Ability to resist flex and vibration
Minimal increase in weight of assembly
Easy automation
Minimal shrinkage upon cure
Excellent cohesive strength
Superior chemical and heat resistance
https://sciencecentre.3mcanada.ca/latest/can-you-use-adhesive-as-an-alternative-to-rivets-and-welds
Greater design potential
Mechanical fasteners can’t accommodate tolerance of light, thin and dissimilar materials
Structural adhesives can bond a larger variety of materials together
Smooth, seamless finish
Increased productivity
Mechanical fastening can require equipment and extra processing time
Saves time and space on the floor
More durable
Rivets and welds could leave behind heat distortion and stress points that can be vulnerable to stress and breakage
Structural adhesives create continuous bonds that disperse force, and don’t damage the substrates
Additional Resources on Structural Adhesives
https://www.compositesworld.com/articles/structural-adhesives-part-i-industrial
https://www.lord.com/products-and-solutions/adhesives/structural/basics-best-practices
Calculating Adhesive Strength Required
What are the modes of failure for adhesive joints:
Fracture mechanics: https://en.wikipedia.org/wiki/Fracture_mechanics
Mode I: Opening mode - A tensile stress normal to the plane of the crack
Mode II: Sliding mode - A shear stress acting parallel to the plane of the crack and perpendicular to the crack front.
Adhesives exhibits the highest resistance to fracture
Mode III: Tearing mode - a shear stress acting parallel to the plane of the crack and parallel to the crack front
Joint resistance is increased by designing geometry that:
The bonded zone is large
It is mainly loaded in mode II
Stable crack propagation will follow the appearance of a local failure
https://www.substech.com/dokuwiki/doku.php?id=adhesive_joints
Lap and strap joints provide the maximal bonding strength
Butt joint is the weakest joint type
Fillet joint imparts additional strength to adhesive joints
Basic rules of adhesive joint design:
Peeling and cleavage stresses should be minimal;
Shear stress is preferrable than tension;
Bonding area should be as large as possible;
Adhesive layer thickness should be minimal and uniform.
https://www.substech.com/dokuwiki/doku.php?id=adhesive_selection
Characteristics of Epoxy Adhesives:
Very good resistance to chemicals (acids, alkalis);
Good resistance to water;
Very good thermal resistance;
Strong durable bonding;
Low shrinkage;
Ability to fill wide gaps;
Brittleness (poor elasticity);
Poor peel strength.
ASC2020 Regs 11.2 - Occupant Weight
Official weight of each occupant will be 80kg
Adhesive Mechanical Design
The design of a bonded joint shall satisfy the following conditions:
The failure of a joint should occur either in the adhesive or in its adherends but not in the interfaces
Allowable shear stress and peeling stress of adhesive are not exceeded
Allowable through-thickness tensile stress and out-of-place shear stress should not be exceeded
The design of adhesively bonded joints shall be validated by either calculation or testing to verify that the design can fulfill the performance requirements.
The limit states shall be satisfied in designing adhesively bonded joints and are provided in Table 10.4
Structural Adhesives
Factors to be considered in adhesive selection:
Adherend materials (such as adhesion compatibility)
Applied loading
Environmental conditions during service
Geometry restrictions
Bonding and curing processes
Costs and other special requirements including health and personnel safety
Partial Safety Factors for Structural Adhesives
The overall safety factor (𝛾m)is the product of four partial safety factors considering the source of adhesive properties (𝛾m,1), the method of adhesive application (𝛾m,2), the type of loading (𝛾m,3), and environmental conditions (𝛾m,4)
𝛾m = 𝛾m,1 x 𝛾m,2 x 𝛾m,3 x 𝛾m,4
Failure Modes and Critical Limit States
A bonded joint, especially the adhesive, may be subjected to the following stresses:
Shear stresses produced by tensile/compressive, torsional, or shear loads imposed on its adherends
Peeling or cleavage stresses at the joint’s edges or corners produced by out-of-plane loads acting on adherends or due to unbalanced banding moment
Out-of-plane tensile/compressive stresses produced by out-of-plane tensile/compressive loads
In-plane tensile/compressive and shear stresses produced by in-plane tensile/compressive and shear loads (depending on joint types)
Out-of-plane tensile, peeling, and cleavage stresses should be evaluated with great care and should be avoided when possible
A main principle in designing bonded assemblies is to ensure that the joint works in shear and to minimize out-of-plane peeling.
For the adhesive layer, a preferred design is one that is primarily stressed in shear or out-of-plane compression
Looking at specifically failure modes for adhesives
Adhesive failure is the rupture due to separation at the adhesive-adherend interface
Failure is mainly caused by insufficient adhesion, due to either material mismatch or inadequate surface treatment
Adhesives are usually considered isotropic
Criterion for cohesive failure of adhesive may be described by maximum stress criterion, that is, the maximum shear or tensile (including peeling) stresses in the adhesive shall be equal or less than the maximum allowable adhesive shear or tensile stresses
Stress Analysis for the Design of Adhesively Bonded Joints
Analysis of the structural behaviour of adhesively bonded joints is often conducted by studying a 2D representation of the joints
Behaviour and strength of a joint can be described by normal stresses and shear stresses.
The local stress distribution within an adhesive joint can be obtained by considering three loads acting in the FRp substrates at the end of the joint: the axial force, the shear force, and a bending moment.
whe n considering stresses in the adhesive joints, the out-of-plane normal (or peeling) stress and shear stress are two dominant components of stress
A 3D analysis requires numerical methods such as finite element analysis.
Additional Resources: